Fluorescent temperature sensor

ABSTRACT

The present invention is a fluorescent temperature sensor or optical thermometer. The sensor includes a solution of 1,3-bis(1-pyrenyl)propane within a 1-butyl-1-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid solvent. The 1,3-bis(1-pyrenyl)propane remains unassociated when in the ground state while in solution. When subjected to UV light, an excited state is produced that exists in equilibrium with an excimer. The position of the equilibrium between the two excited states is temperature dependent.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to thermometry and moreparticularly to a fluorescent temperature sensor comprising an ionicliquid solvent and dissolved fluorescent material capable of forming anexcimer.

BACKGROUND OF THE INVENTION

Real time temperature monitoring is conducted in an industrial settingfor process optimization, waste minimization, and energy conservation.For example, precise and real time in-vivo temperature monitoring may beused in applications such as biomedical and cancer diagnosis and duringhypothermia therapy or surgery where temperature fluctuations of even afew degrees can create problems or even be life-threatening to apatient. Despite the commercial importance of this technology, thedevelopment of molecular temperature sensors has been inadequate.Although conventional contact techniques such as liquid-in-glassthermometers, thermistors, thermocouple taps, and resistance temperaturedetectors (RTDs) have their place, employing light as the informationcarrier rather than heat has several benefits. Optical temperaturesensors, which are often referred to as “optodes” or “optrodes”, may bedeployed in situations where it is undesirable or impossible for a wireconnection, to measure temperature at a location having excessiveelectromagnetic noise, to monitor temperature in a corrosive environmentor an explosion and importantly, to monitoring the temperature ofhigh-speed moving parts (turbine blades, for example).

Non-contact optical approaches are also important because they providetemperature measurements with high spatial resolution, and are useful inmapping temperature for applications that require high spatialresolution, at the cellular level for example, in microfluidic chips andmicroelectromechanical systems (MEMS), and for locating heat“bottlenecks” in integrated circuits, and as temperature monitors formulti-well plates used in biology and combinatorial chemistry.

Remote two-dimensional infrared thermography has been used for measuringtemperature. While this technique offers some of the advantages ofnon-contact approaches, it is limited by the strong absorption ofradiation by water vapor and glass. Importantly, few objects trulybehave like blackbodies, and therefore radiation from a solid objectseldom exhibits a distinctive thermal signature. By contrast,luminescent signals, which are also multidimensional, offer sensitive,selective, and rapid feedback. Luminescence is often the observable ofchoice of chemosensors and molecular-level devices.

In the prior art, luminescent temperature sensors measure temperatureusing the temperature-dependent decay times of excited states ofmaterials, intensities, excitation and/or emission wavelength maxima.There remains a need for better luminescent thermometers.

OBJECTS AND SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a robust, precise and broad-ranged ratiometric luminescentthermometer that uses the temperature-dependent excited-stateequilibrium of a novel dual fluorescent reporter and ionic liquid. Thepresent thermometer employs the unique thermofluid properties of anionic liquid; namely, its high thermal coefficient of viscosity andbroad-liquidus range. Because the associated wavelengths are wellresolved, dispersive optics are no longer necessary. As a result, thepresent invention is well adapted for inexpensive and mobile formats. Inaddition, by implementing confocal or multi-photon methods, highlylocalized excitation is made possible thereby adapting the device foruses that include, but are not limited to, on-chip PCR and laser-basedT-jump calibration. Further, the present invention may be adapted forutility as a temperature sensitive paint (TSP) by entrapping the ionicliquid within thin transparent sol-gel films. In industrial settings,this embodiment of the present invention would adapt itself to a widearray of designs including coatings on, for example, wings or turbines,to provide a complete temperature-based spatial map that may be used toidentify areas of metal fatigue.

In summary, the present invention is a fluorescent temperature sensor oroptical thermometer comprising a solution of 1,3-bis(1-pyrenyl)propanewithin a 1-butyl-1-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ionic liquid solvent. The1,3-bis(1-pyrenyl)propane remains unassociated in the ground state whilein solution. When subjected to UV light, an excited state is producedthat exists in equilibrium with an excimer, the position of theequilibrium between the two excited states being temperature dependent.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates emission spectra for an optical thermometer accordingto the present invention;

FIG. 2 illustrates an analytical working curve for an opticalthermometer according to the present invention; and

FIG. 3 is a chemical equilibria expression showing the formation of anexcimer of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a fluorescent temperature sensor or opticalthermometer adapted to function in a variety of formats. The moleculartemperature sensor comprises a solution of 1,3-bis(1-pyrenyl)propanewithin an ionic liquid solvent; namely, 1-butyl-1-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide. The 1,3-bis(1-pyrenyl)propane remainsunassociated in solution during its ground state. However, whensubjected to UV light, an excited state is generated that co-exists inequilibrium with an excimer formed within the solution. The position ofequilibrium between the two excited states is temperature dependent andtherefore functions to accurately convey the temperature of any objecton which the solution is coated or otherwise in contact with. The uniquethermofluid properties of the ionic liquid, namely its high thermalcoefficient of viscosity and broad liquids range allow the thermometerto be adapted to a wide variety of application.

FIG. 1 illustrates emission spectra of the present invention normalizedto the intensity of the monomer band at |₃₇₆=1.00 for a 5 μM solution of1,3-bis(1-pyrenyl)propane within 1-butyl-1-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ionic solution throughout a typicaltemperature cycle of the present invention. As shown in the graph, thespectral response of the 1,3-bis(1-pyrenyl)propane/ionic liquid systemis strongly governed by temperature. Under controlled heating, theexcited-state equilibrium is shifted in favor of excimer formation and aprominent band near 475 nm is shown to result. The excitation wavelengthof the solution is 325 nanometers (nm). The solid lines indicate spectrarecorded during the cooling portion of the cycle. Spectra 1 through 9correspond to the following temperatures in degrees Celsius (° C.): 30,40, 50, 60, 70, 80, 90, 100,120.

Turning to FIG. 3, formation of the excimer of the present invention issummarized by a chemical equilibria expression. The bottom portion ofthe expression shows 1,3-bis(1-pyrenyl)propane in the ground state. Withthe appropriate light excitation, the ground state is transformed intoan excited state, shown in the top portion of the expression where thedark circles indicate one of the pyrenyl groups in an excited state.This excited state rearranges in space to create the excimer, which isshown in braces in the top right portion of the equilibrium expression.Equilibrium constants k1, k−1, klM, kM, kE, and Kie correspond to theelementary steps that connect the ground states. The excited state isshown on the top left, and the excimer is shown on the top right. Uponcontrolled heating, the excited state equilibrium shifts in favor offormation of the excimer and a prominent band near 475 nm results asshown in FIG. 1.

The unnormalized emission spectral data of the present inventionexhibits an isoemissive point (wavelength where the intensity does notchange with temperature) at 446 nm for temperatures greater than orequal to 90 degrees Celsius which corresponds to the chemicalequilibrium expression model depicted in the FIG. 3. Although a singleisoemissive point is not essential to operation of the sensor, in thepresent example it does exist.

The response of the optical sensor according to the present invention totemperature is reversible. As shown in FIG. 2 of the drawings, theresponse is two-color and ratiometric. That is, the luminescent signalat one color is normalized to the intensity at a different color. Theratiometric sensor of the present invention is self-referencing andtherefore highly useful. That is, the values for the ratio operateindependent of total signal, optical path or amount of probe moleculeand therefore are less subject to error. The ratio is independent of theintensity of the illumination source applied, the optical configuration,or the luminophore concentration.

For example, intentional photobleaching of the solution of the presentinvention by subjecting it to continuous UV irradiation (24 hours, 10milliwatts (mW)) at 60 degrees Celsius resulted in a decreased intensityat the isoemissive point of the solution by 15 percent while the ratioIE/IM changed by less than 0.9 percent. This corresponds to an error intemperature estimation of the present invention of better than 0.35degrees Celsius.

The reversibility and robustness of the temperature sensor of thepresent invention are demonstrated by the fact that repeated exposure toheating and cooling cycles from 60 degrees Celsius to 90 degrees Celsiusover a period of 8 hours resulted in mean deviations from the workingcurve shown in FIG. 2 of only 0.16 degrees Celsius (at a temperature of60 degrees Celsius) and a 0.30 degrees (at a temperature of 90 degreesCelsius).

In contrast to prior art excimer/exciplex based molecular thermometers,the upper operational range of the sensor of the present invention isnot limited by the boiling point of common organic solvents but instead,depends upon the thermal stability of the fluorophore employed. As aresult, the operable range for a sensor according to the presentinvention may extend beyond the range of 25 degrees Celsius to 140degrees Celsius.

The present invention therefore provides a robust, precise, broad range,ratiometric, luminescent temperature sensor that provides atemperature-dependent, excited state equilibrium of a dual fluorescentreporter within an ionic liquid solvent. The sensor exhibits thethermofluid properties of the ionic liquid employed, namely its highthermal coefficient of viscosity and broad liquids range. Modificationof the absolute viscosity of the solution by changing the ionic liquidmay therefore be used to configure the sensor to a specific application.Since the associated wavelengths are well resolved, dispersive opticsare no longer necessary, facilitating adaptation of the presentinvention to both inexpensive and mobile formats. Implementing confocalor multi-photon technology to subject the sensor to light can providehighly localized excitation thereby adapting the sensor to uniqueutilities such as on-chip PCR or laser based T-jump calibration. If theionic liquid of the present invention is encased within thin transparentsol-gel films, nanoscale liquid-in-glass sensors may be provided in theform of temperature sensitive paints.

The sensor of the invention is adapted for use for a wide variety ofindustrial processes. For example, the ionic liquid and fluorophore ofthe present invention may be disposed in between an inner pipe and atransparent outer pipe forming a concentric sensor. The temperaturealong all points on the pipe may then be monitored by shining UV lighton the pipe to obtain a complete temperature-based spatial map.Similarly, wings or turbines may be coated with a film of transparentsol-gel that has been embedded with the fluorescent composition offluorophore dissolved within the ionic liquid. The film will thenprovide a spatial map of temperature for identifying potential points offatigue. The ability to spatially amp temperature can also be used inconjunction with multi-well plates to monitor temperature in multiplewells. This could be done by using a thin film of fluophor in ionicliquid on the bottom of the multi-well plate and sealing with atransparent cover.

While this invention is described as having a preferred design, it isunderstood that it is capable of further modification, uses and/oradaptations following in general the principle of the invention andincluding such departures from the present disclosures as come withinknown or customary practice in the art to which the invention pertains,and as may be applied to the essential features as set forth and fallwithin the scope of the invention or the limits of the appended claims.

1. A composition of matter useful as a temperature sensor, said sensorcomprising at least one fluorescent compound capable of forming anexcimer and 1-butyl-1-1-methyl pyrrolidiniumbis(trifluoromethylsulfonyl)imide ionic liquid solvent, said at leastone fluorescent compound capable of forming an excimer is dissolvedwithin said imide ionic liquid solvent to provide a fluorescent solutionthat is adapted to convey a temperature dependent luminescent signalupon UV irradiation.
 2. A sensor as in claim 1 and wherein said at leastone fluorescent compound contains a pyrenyl group.
 3. A sensor as inclaim 2 and wherein said one fluorescent compound is1,3-bis(1-pyrenyl)propane.
 4. A sensor as in claim 1 and furthercomprising a light transmissive container, said fluorescent solution iscontained within said light transmissive container.
 5. A sensor as inclaim 1 and further comprising a transparent sol-gel material, whereinsaid fluorescent solution is embedded within said sol-gel material.
 6. Asensor as in claim 1 and further comprising an ultraviolet light source,said ultraviolet light source is adapted to irradiate said fluorescentsolution.
 7. A sensor as is claim 6 and further comprising a detector,said detector for detecting a light signal generated by said fluorescentsolution following irradiation by said ultraviolet light source.
 8. Acomposition of matter useful as a temperature sensor, said sensorcomprising 1,3-bis(1-pyrenyl)propane and 1-butyl-1-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide, said 1,3-bis(1-pyrenyl)propane isdissolved within said 1-butyl-1-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide to provide a fluorescent solution thatis adapted to convey a temperature dependent luminescent signal upon UVirradiation.
 9. The transparent sol-gel material as in claim 5 whereinsaid sol-gel material is disposed as a thin film and used as a coating.10. The transparent sol-gel material as in claim 5 wherein, when saidsol-gel material is disposed as a thin film and encapsulated as smallliquid-in-glass sensors, said sensors are provided as an additive in theformulation of temperature sensitive paints.
 11. The transparent sol-gelmaterial as in claim 9 wherein, when said thin film is deployed as acoating, it is sealed with a transparent cover.